The identification and quantitation of human cytotoxic cells are basic for the understanding of the immune response to allo-antigens, intracellular infectious agents and tumor antigens, and might acquire clinical relevance for the evaluation of the patient's immune status. We propose to investigate some aspects of cell-mediated cytotoxicity in humans: 1) to analyze the phenomenon of interferon induction in mixed cultures of lymphocytes and virus infected or tumor-derived cells; 2) to investigate the enhancing effect of interferon and interferon inducers on natural killer cells; 3) to study the mechanism of interferon-induced resistance to lysis of target cells; 4) to identify cytotoxic cell types generated during an immune response; 5) to analyze the specificity and the genetic control of cytotoxic T cells against virus infected and transformed cells; 6) to produce by somatic cell hybridization monoclonal antibodies against lymphocyte surface antigens able to identify lymphocyte subpopulations.